The following processes are typical of the manufacture of a solid propellant rocket motor: (1) fabrication of the motor case, (2) insulation of the interior of the case, (3) application of a "liner" to the surface of the insulation to ensure bonding to the propellant grain, (4) casting of the uncured propellant into the motor and against the liner, and (5) curing the propellant grain.
The motor case, which serves as a pressure vessel, is constructed of lightweight material (metal or fiber composite) that provides adequate strength only at relatively low temperature. The function of the insulation is to protect the case from being weakened by exposure to the hot gases formed during combustion of the solid propellant. Generally the insulation is applied to the interior of the motor case in the form of an uncured elastomeric material such as ethylene-propylene (EPDM) or nitrile rubber containing particulate, solid, refractory filler. The insulation is then cured, generally under pressure by application of moderate heat.
The composition of the liner and techniques for its application depend in part on the nature of the insulation but mainly on the composition of the propellant that will be cast against it. These matters are discussed in greater detail below.
The uncured propellant typically consists of a viscous suspension of particulate solids (e.g., ammonium perchlorate oxidizer, nitramines, powdered aluminum fuel, and minor amounts of combustion modifiers) in a curable elastomeric binder (e.g., Hydroxy terminated polybutadiene). After casting, the propellant is cured, usually for about one week at temperatures below about 70.degree. C. During cure the propellant becomes a strong, elastic solid that adheres tightly to the liner-coated insulation.
The rate at which combustion gases are formed during burning of the propellant grain is proportional to the area of the grain that is exposed. In turn, the chamber pressure depends on the balance of volume of gases generated and gases discharged through the rocket nozzle. Thus, to achieve the designed pressure/thrust history, and particularly to avoid catastrophic overpressure, it is essential that during combustion the propellant grain remain securely bonded to the insulator wherever they are in contact. The bond must be strong in tension and resistant to peeling.
Such bonding is achieved by application of an adhesive liner to the surface of the insulation prior to casting the uncured propellant. The adhesive liner may take the form of a thin layer of filled, curable, liquid elastomer (commonly the same elastomer that is used as the binder of the propellant) or, alternatively, a thinner, unfilled liquid layer consisting of a diluted or undiluted polyfunctional substance capable of bonding chemically to the elastomeric binder of the propellant. In the case of a hydroxy-functional binder, such as hydroxyl terminated polybutadiene (HTPB), the substance of choice is a polyfunctional isocyanate similar or identical to the binder curative of the propellant formulation.
Typically liners are applied to the insulation mechanically, i.e., by brushing or spraying. Neither technique permits precise control of liner thickness and hence the properties of the resulting bond. Further, spraying requires that the liner formulation be of low viscosity, and this generally requires addition of a diluent. Suitable diluents are either flammable (e.g., hydrocarbons, ketones, and esters) or environmentally hazardous (halohydrocarbons).
Therefore, what is needed in this art is a method for improving the means of bonding propellant to insulation to achieve a high-quality, uniform bond without incurring fire or environmental hazards.